Electrical submersible well pumps for deep wells are normally installed within casing on a string of tubing. Usually the tubing is made up of sections of pipe which are screwed together. The motor is supplied with power through a power cable that is strapped alongside the tubing. The pump is typically located above the motor and connected to the lower end of the tubing. The pump pumps fluid through the tubing to the surface. One type of a pump, a centrifugal pump, uses a large number of stages and is particularly suited for large pumping volume requirements.
For lesser pumping volume requirements, a progressing cavity or PC pump may be employed. PC pumps utilize a helical rotor that is rotated inside an elastomeric stator which has double helical cavities. PC pumps may be surface driven or bottom driven. Surface driven PC pumps have a rod which extends down to the pump in the well, whereas bottom driven PC pumps are driven by electric motors located in the well.
PC pumps are widely used in applications where significant quantities of solids, such as sand and scale, are likely to be encountered. When a large volume of solids enter the pump, the pump may not be able to remove the solids, causing the pump to lock up. Lock up can also occur if the pump assembly shuts down for any reason. The solids in the tubing string settle back down on top of the pump, again causing it to lock up. When this situation occurs on a standard surface-driven PC application, the rod string is pulled from the well bringing the pump rotor with it. The tubing and pump stator are then flushed and circulated until they are clean before the pump rotor and rod string are reinstalled into the pump stator. Bottom-driven PC pumps present a significant drawback to accomplishing this procedure. The same conditions that lock-up surface driven applications also apply to the bottom drive systems.